Any vacuum leakage in the tins or pushrod covers will cause this. Also check and make sure you don't have Eastern brand tappet lifters. They are easily identifiable by holes in the top of the lifter body, once you have pulled pushrod covers. For some reason they decided to put holes in the top and bottom of the lifter body, but plugging the top ones is all you need to worry about, or buy a set of VTwin lifters. Make sure to mic the body and bore of the tappet holders, since extra clearance there will cause drain down problems. Also check breather timing to make sure it's correct. Hope that helps.